A flame scanner monitors the combustion process in a fossil fuel fired combustion chamber to provide a signal indicating the presence or absence of a stable flame. With the presence of a stable flame, fossil fuel continues to be fed into the combustion chamber of the steam generator. In the event that the flame becomes unstable, or the flame is lost completely (known as a flame out condition), the flame scanner provides a loss of flame signal. Based upon a loss of flame signal, fossil fuel delivery to the combustion chamber can be discontinued before an undesirable unstable operating condition or flame out condition develops. In some systems, a human operator interrupts the fuel supply based upon the loss of flame signal; in other systems a burner management system (BMS) interrupts the fuel supply based upon the loss of flame signal.
Conventional flame scanners produce an electrical signal based upon a monitored flame. This resulting analog electrical signal is transmitted to processing electronics that are housed separately from the flame scanner, typically in an equipment rack located adjacent to a control room. The strength of the produced signal is typically proportional to the intensity of the monitored flame. If the signal strength falls below a lower set point, or rises above an upper set point, delivery of main fuel into the combustion chamber is interrupted. Set points are sometimes referred to as trip points.
A flame scanner collimator body is a housing that shields optical components that gather light from burner flames. The optics within this housing focuses the light from the burner flames on a transmission medium, such as a fiber optic cable, to transmit the light for flame analysis outside a combustion chamber of the boiler away from the hot burner zone. More specifically, one end of the fiber optic cable terminates in a fixed lens barrel assembly disposed within the collimator body. However, there are problems associated with existing collimator bodies.
For example, the lens barrel assembly is fixed to the collimator body using a plurality of screws. There is no relief of tension on the cable fibers of the fiber optic cable if there is any tension between the cable and lens barrel assembly. This tension breaks delicate fibers thus reducing the light transmission to the flame intensity and frequency analysis circuitry outside the boiler.
Conventional lens barrel assemblies include relatively large lens barrels disposed within the collimator body. However, large lens barrels reduce the cooling airflow through the collimator body, which causes excessive heat build up between the junction of the lens barrel and the fiber optic cable. Excessive heat causes the bonding material that holds the fibers in place to weaken, thus causing the fibers to pull back from the end of the cable and retreat from the lens barrel focal point thereby reducing the light power transmission therethrough.
All hardware in the field environment where the flame scanner resides requires maintenance at one time or another. Therefore, the collimators are commonly disassembled and reassembled when the occasion arises requiring parts cleaning or replacement. However, the typical field repair/cleaning takes place on a hot collimator where each collimator is typically assembled using four (4) to seven (7) hex head set screws and the technician is wearing gloves and/or standing over floor grating, thus presenting a greater risk of loosing the screws. Some of these set screws hold the lens barrel assembly in the collimator body as discussed above.
Lastly, the relatively large lens barrel disposed within the collimator tube creates a large pressure drop that restricts cooling/purge airflow over the lens. The result is that dust born in the cooling airflow deposits on the lens over time much like dirt depositing on the rear window of an SUV or station wagon.
Accordingly, a need exists for a flame scanner collimator body that has a reduced parts count to reduce assembly cost and provide for easier maintenance. A need also exists for a collimator body that has a mechanically rugged design for increased protection of internal components and that improves cooling/purge airflow to improve cleaning action on a lens barrel lens.